The present invention relates, most generally, to a process and apparatus for manufacturing semiconductor devices. More particularly, the present invention is directed to forming sub-wavelength size contact openings using attenuated phase-shift photomasks.
Attenuated phase-shift masks (APSM""s) are used extensively in small geometry semiconductor manufacturing to achieve process latitude and pattern resolution greater than achievable using conventional binary photomasks. In today""s rapidly advancing semiconductor manufacturing industry, which includes increasingly high levels of integration and correspondingly small feature sizes, the use of APSM""s is essential in the execution of state-of-the-art fabrication processes. APSM""s are fabricated by replacing the opaque part of a conventional mask with a halftone filmxe2x80x94one that is partially transmissive. The transmissivity of such a halftone film is generally on the order of about 10% transmission. The halftone film is chosen to desirably shift the phase of the light it transmits by 180 degrees. The light which passes through the clear area of the APSM, in contrast, is not phase shifted. In this manner, destructive interference occurs between interfering light which is out of phase.
One of the main problems with APSM""s, however, is the printing of sidelobes, which are unwanted images in the final pattern caused by constructive interference between adjacent clear features in the photomask pattern when the adjacent clear features are closely spaced and are separated by a distance on the order of the exposure wavelength. The occurrence of sidelobes introduces spurious ring structures and windows in dense patterns, and is highly sensitive to the presence of any aberration and defocusing in the imaging system. Sidelobe printing is especially problematic for hole dimensions less than 0.5xc3x97xcex/NA (where xcex is the wavelength of light and NA is the numerical aperture of exposure tool), where high transmission masks must be used, and for 193 nm lithography, where current photoresists have insufficient surface inhibition to prevent sidelobe formation.
Several techniques have been used to attempt to alleviate the problem of printing unwanted features such as sidelobes. Each of these techniques includes associated shortcomings. One popular technique is underexposure. Using the underexposure technique, the pattern is printed at a lower dosage such that unwanted sidelobe features, which have a lower intensity than the main features, are not printed. A shortcoming associated with the use of this underexposure technique is that the main features are printed in smaller dimension. Additionally, the critical dimensions (CD""s) are more sensitive to mask error and the depth of focus (DOF) is generally lower. Another technique is the RIM-type phase-shift mask. In RIM-type phase-shift masks, a layer of opaque material, usually chrome, is used to block out selected regions of the pattern such that the unwanted features are suppressed. A shortcoming of the RIM-type phase-shift mask is that such masks are more expensive than conventional APSM masks because of the additional complexity associated with designing, creating and aligning the chrome structures.
A third technique involves the use of auxiliary patterns as additional features on the mask, that, when properly designed and placed, suppress the intensity of the sidelobes. The auxiliary patterns may be rectangular transmission areas between closely spaced contact windows or a grid pattern for an array of contact windows. The auxiliary patterns are suitable for periodic patterns but cannot easily be generalized to non-periodic patterns. Generally speaking, auxiliary patterns occupy extra real estate on the mask and affect all of the features which surround the auxiliary pattern. A fourth technique, used to try to alleviate the problem of printing unwanted features such as sidelobes, is the manipulation of the illumination conditions. In general, interference decreases with decreasing coherence of the light waves. Lowering the coherence of the illumination sources can reduce the intensity of sidelobes but cannot eliminate sidelobe problems completely.
According to conventional photomask patterns, contact openings are produced in semiconductor devices by providing square apertures on the photomask to define a circular contact opening in the photo-pattern created in the photoresist at the wafer level. The present invention addresses shortcomings of the techniques in the prior art which attempted to prevent sidelobe formation, and presents clear contact opening apertures on attenuated phase-shift photomasks which prevent sidelobe formation even when the shape-engineered apertures are formed in tightly packed arrays.
The present invention provides an attenuated phase-shift photomask including contact structures which are non-rectangular apertures for forming contact openings on semiconductor substrates. The apertures each have a shape including multiple vertices. The present invention also provides an attenuated phase-shift photomask having multiple non-rectangular contact structures, in which the contact structures are formed of a transmissive material and bounded by a partially transmissive area, and include vertices which extend outwardly. When the array is used to form a corresponding array pattern in a photosensitive material, sidelobe formation is suppressed.
Another aspect of the present invention is a method for forming contact openings on a semiconductor substrate. The method includes the steps of forming a photosensitive film over a substrate, providing an attenuated phase-shift photomask including non-rectangular transmissive figures, each including outwardly extending vertices, and exposing contact areas on the photosensitive film by positioning the photomask in fixed position with respect to the substrate and providing a light source opposite the surface of the photomask which faces the substrate, such that each of the exposed contact areas correspond to a transmissive figure.
Another aspect of the present invention is the pattern formed in a photosensitive material. An exemplary pattern includes an array of tightly packed contact holes of small dimension, and the array is characterized by the absence of pattern defects interposed within the array.
Another aspect of the present invention is a computer readable medium. The computer readable medium includes a set of instructions for generating an attenuated phase-shift photomask having a pattern including non-rectangular contact structures, each contact structure being formed of a transmissive material, bounded by a partially transmissive area, and including outwardly extending vertices.
The present invention also provides a method for generating an attenuated phase-shift photomask. The method includes the steps of providing a photomask manufacturing tool and a computer electronically connected to the photomask manufacturing tool, providing a computer readable medium including instructions for generating an attenuated phase-shift photomask having non-rectangular contact structures, each including a plurality of outwardly extending vertices and formed of a transmissive material, and engaging the computer readable medium with the computer, such that the computer reads the computer readable medium and communicates with the photomask manufacturing tool and electronically instructs the photomask manufacturing tool to generate the attenuated phase-shift photomask.
It is to be understood that both the forgoing general description and the following detailed descriptions are exemplary, but not restrictive, of the invention.